Download Catalysis

Catalysis
Chemistry of the
Elements
Greenwood & Earnshaw 2nd Ed.
Heterogeneous Catalysis
Advantages
Disadvantages
• Easy Recovery of
catalysts. i.e. filtration
•
•
•
•
Poor heat dissipation
Surface limited
Easily poisoned
Not stereospecific
Homogeneous Catalysis
Advantages
Disadvantages
• Reaction occurs
• More difficult catalyst
throughout solution.
recovery.
• Better heat dissipation
• Faster reactions – not
surface limited
• More specific –
stereospecific –
assymmetric induction
• Not as easily poisoned
1
Homogeneous Catalysis
Most homogeneous catalytic reaction
mechanisms involve a) a series of
oscillations between 16 and 18 valence
electrons (NVE) on the transition metal
center, and b) a series of transformations
between coordinatively unsaturated and
saturated geometries involving
coordination numbers of 3, 4, 5, 6
Hydrogenation – Wilkinsons Catalyst
RhCl 3 + 3 PPh3
Ph3P
hot
ethanol
P = PPh3
RhCl(S)P2
[NVE 16]
PPh3
Rh
Ph3P
[NVE 16]
Cl
H
S = benzene-ethanol
H2
-S
RhCl(H)2P2
C
H
Ph3P
C
Rh
[NVE 16]
C
Ph3P
Cl
Conditions:
25 EC, 1 atm H2
C
[NVE 18]
H
HC CH
Ph3P
+S
Rh C CH
Ph3P
[NVE 16]
Cl
Hydrogenation – Wilkinsons Catalyst
•Ease of hydrogenation: terminal and
cyclic olefins > geminally substituted >
internal and conjugated olefins > more
highly substituted olefins > cyclic dienes.
•Assymmetric hydrogenation possible with
chiral phosphines (PR,R’,R”). Optical
purities range from a few percent to 85%.
2
Hydroformylation – Oxo Process
•Very Important: production 109 Kg/yr.
•Olefin Reactivity: unbranched terminal >
unbranched internal > branched terminal
> branched internal & cyclic olefins.
•Cobalt catalyst produces branched and
unbranched aldehydes and also alcohols.
•Newer Rhodium catalyst gives mostly naldehydes at high phosphine concentration
Hydroformylation – Oxo Process
O
CH3CH CH2 + CO
+
CH3CH2CH2C
H2
General Reaction:
H
H2
CH3CH2CH2CH2 OH
Catalyst Formation:
2 Co(OAc)2 + 8 CO + 2 H2
[Co2(CO)8]
H2
More selective for terminal olefins
formation of alcohols.
P(Bu)3
2 HCo(CO)3P(Bu)3
2 HCo(CO)4
Union Carbide Catalyst:
Gives mostly n-aldehydes at high
Ph3P concentrations: 2:1 at 10% PPh3
15.3:1 in pure PPh3.
[HRh(CO)(PPh3)3]
Hydroformylation – Oxo Process
O
C C
Co C O
C
O
H
O
O
- CO
+ CO
C
C
O
[NVE = 18]
C
Co
RCH2 CH2
H
C
Co
O
C
O
CH3
[NVE = 16]
C
O
- CO
+ CO
Co
H2
+
RCH2CH3
CH2CH2R
C
regenerated
catalyst
H2
CH2
1,2 Shift or
1,2 addition
R
HC
R
CH
[NVE = 18]
[NVE = 16]
1,2 Shift or
1,2 addition
O
O
C C
Co
C
O
H
O
O
O
C
[NVE = 16]
C
O
- CO
+ CO
3
Hydroformylation – Oxo Process
O
H
C
regenerated
catalyst
Co
C
+
H
C
O
- CO
O
CCH2CH2R
CH2CH2R
O
desired product
+ CO
C
O
O
Co C O
C
[NVE = 18]
C
reductive
elimination
O
alkyl migration
"CO insertion"
O
O
CCH2CH2R
H
Co
O
C
C
O
H
O
oxidative addition
[NVE = 18]
C
O
O
[NVE = 16]
Co
H2
C
CCH2CH2R
C
C
O
Hydroformylation – Oxo Process
- CO
+ CO
R
O HC CH3
C
Co C O
C
[NVE = 18]
O
C
+
Co
C
O
reductive
elimination
alkyl migration
"CO insertion"
O
R
C CH
C
H CH3
Co
[NVE = 18]
C
O
C
H
O
O
R
C CH
C
CH3
Co
C
[NVE = 16]
O
C
O
O
H2
oxidative addition
Oxo Production of Alcohols
O
H
C
Co
C
H
O
C
Co adds to O of C=O
Co
CCH2CH2R
+
O
C
O
OCH2CH2CH2R
[NVE = 16]
C
C
O
O
H2 oxidative addition
Co adds to C of C=O
OH
O
C
Co
O
regenerated
catalyst
C
O
branched product
O
O
H
C
CH C H
CH3
R
O
O
O
C
O
CHCH2CH2R
C
[NVE = 16]
C
O
O
C
H
H2 oxidative addition
OH
CHCH2CH2R
C
H
reductive
Co
elimination
C
C
O
O [NVE = 18]
H
OCH2CH2CH2R
H
Co [NVE = 18]
C
O
reductive
elimination
O
HOCH2CH2CH2R
+
regenerated catalyst
4
Carbonylation of Methanol
CH3OH
Net Reaction:
O
CH3COH
+ CO
PCO = 16 atm, T = 175E
EC
solvent: benzene, water, acetic acid, methanol,
I
O
C
Rh
O C
I
I
CH3I
I
O C
[NVE = 16]
[NVE = 18]
C
I
CH3OH
I
O C
C CH
3
+ CO
Rh
O
[NVE = 16]
C CH3
C
[NVE = 18]
O O
Desired product
1.4 x 109 Kg/yr (1978)
I
Rh
O C
I
+ O
CH3COH
I
monomer
& dimer
I
CH3I
"insertion"
alkyl migration
CH3
O
Reductive
elimination
O
CH3C I
I
Rh
Slow
Oxidative addition
Cyclic Trimer & Tetramerization of Alkynes
O
Reppe
Synthesis:
O
Ni
O
O
O
O
S
Ni
O
S = Solvent: tetrahydrofuran or dioxan
Chelate: Acetylacetone or salicaldehyde
S
O
O
PPh3
O
O
Ni
PPh3
O
Ziegler-Natta Catalysis
alkane
solvent
TiCl4 + Al 2Et6
H2C CH2
LnTi
Et
H2C
CH2
H2C CH2
LnTi
LnTi
LnTi-Et
Ti(III)EtnCl3-n
brown suspension
Classical Mechanism:
CH2CH2
Et
LnTi
LnTi CH2
CH3
CH2CH3
CH2CH2
Etc.
CH2CH2
LnTi
H2C CH2
CH2CH3
H2C CH2 CH CH
2
3
H2C
CH2CH2
LnTi
LnTi CH2CH2
H2C CH2
CH2CH2
or
Et
CH2
H2C CH2
H2C
CH2CH3
CH2
CH2CH2
LnTi
CH2CH3
H2C CH2
5
Ziegler-Natta Catalysis
TiCl4 + Al 2Et6
alkane
solvent
H
CH2
HCH
LnTi-Et
Ti(III)EtnCl3-n
brown suspension
Carbene Mechanism:
H
CH2
H
CH2
HC H
HC H
Ti
H2C
Ti
H
H2C CH2
H2C
CH2
H
CH2
Ti CH
H
CH2
Ti
Etc.
H
CH2
H2C CH2
H2C
H2C CH2
H CH2
Ti CH
CH2
CH2
H
H2C CH2
H2C CH2
H CH2
Ti CH
CH2
Ti
H
CH2
HC H CH2
Ti
CH2
CH2
Ziegler-Natta Catalysis
Classical mechanism requires chain transfer of
an ever increasing length chain, entropy
consideration as polymer chains become very long.
The “concerted pathway” & the carbene mech.
was invoked to counter the entropy troubles above.
Polypropylene is only “head-to-tail”, most
valuable polymer produced by Z-N catalysis. The
methyl groups alternate along the chain.
Reactivity: terminal > geminal > internal olefins.
Only homopolymerization feasible.
Olefin Metathesis
Net Reaction:
2 CH3CH CH2
Catalysts:
M CH2
H2C CH2
+
O
M = (Et3P)2WCl 2 , W(CO)5 ,
CH3CH CH2
CH3CH CHCH3
Others based on
Mo, Re, W
CH3CH CH2
CH3CH CH2
M CH2
CH2
M
CH3CH CHCH3
CH3
CH3CH CH
M
CH2
CH3CH
M
CH3
CH
CH2
CH3CH CH2
CH3CH
M
+
CH2
CH2
6
Olefin Metathesis
Polymerization
O
Catalysts:
Others based on
Mo, Re, W
M = (Et3P)2WCl 2 , W(CO)5 ,
R
HC
R
HC
R
HC
M
M
M
R
C
H
R
HC
M
etc.
M
R
HC
R
HC
R
HC
M
M
M
Olefin Metathesis
•Allows versatility of olefin feed to
industry, principally ethylene, propylene.
•Allows polymerization using cyclic olefins.
•The cyclic olefin ring size allows variable
spacing of unsaturations along the polymer
chain in the resulting metathesis polymers.
C1 Catalysis, Syn-fuels & Coal Gasification
1500E
EC
H2O + C
Lurgi Process:
H2 + CO
"Water Gas" Reaction
"Shift" Reaction: CO
+
H2O
Fischer-Tropsch Chemistry:
Syn-gas:
CO
+ 3 H2
"Sasol" Process:
400E
EC
H2 + CO2
Iron Oxide Catalyst
Group 8 metals are important in catalysis,
mechanisms are complex/unknown
Ni
CH4 + H2O
325E
EC
n CO + 2n H2
Co
CH2
n
+ n H2O
Syn"crude" = gasoline, greases, etc.
Mobil Process:
CO
+ 2 H2
Cu/ZnO
250E
EC, 50 atm.
CH3OH
Fuel, Feedstock
Patented 1977, Mobil "ZSM-5"
based on framework silicates
Syn"crude" = gasoline, greases, etc.
Conservation of "remote" natural gas:
CH4 + H2O
CO + 3 H2
CH2
n
Cu/ZnO
250E
EC, 50 atm.
+ n H2O
CH3OH
7